Answers

사랑했다 is the past tense verb for 'love' and -는데 is a contrasting conjunction used with a present and past tense verb. So, 얼마나 사랑했는데 means 'How much I loved you'.

하나 means 'one', and 에 has many uses, 'by, on, with, at, in, because of, etc etc', . It here is used as the sense of "because of". Well, 너 하나에 울고 웃고 can't remain well in English, like 'I cry and smile at you'.

-도 means 'even if', so 사랑에 울어도 사랑에 아파도 = 'even if I cry at love and I get hurt by love'.